▎ 摘　　要

Although grain boundaries (GBs) in two-dimensional (2D) materials have been extensively observed and characterized, their formation mechanism still remains unexplained. Here a general model has reported to elucidate the mechanism of formation of GBs during 2D materials growth. Based on our model, a general method is put forward to synthesize twinned 2D materials on a liquid substrate. Using graphene growth on liquid Cu surface as an example, the growth of twinned graphene has been demonstrated successfully, in which all the GBs are ultra-long straight twin boundaries. Furthermore, well-defined twin boundaries (TBs) are found in graphene that can be selectively etched by hydrogen gas due to the preferential adsorption of hydrogen atoms at high-energy twins. This study thus reveals the formation mechanism of GBs in 2D materials during growth and paves the way to grow various 2D nanostructures with controlled GBs.